The general objective of this project is to determine the effects of long-term alterations in use on spinal pathways in cat and rat. Four procedures will be employed: tenotomy, local anesthesia, spinal section, and pharmacological denervation. Studies will concentrate on two synapses that between 1A afferents and alpha motoneurons and that between motoneuron recurrent collaterals and Renshaw cells, using both population responses (homonymous and heteronymous monosynaptic reflexes) and extracellular unit recording. Available data on chronic use and disuse are difficult to interpret because, generally, neither the site (s) nor the nature of the change in synaptic efficacy, nor the alteration in use which produced the change, has been clearly identified. The proposed methodology will minimize the above problems: Employing the occurrence of heteronymous monosynaptic reflexes to indicate increased synaptic efficacy, experimental animals will be compared to unoperated controls, avoiding inter-animal comparisons where changes on the experimental side can not be distinguished from opposite changes of the contralateral "control". Employing chronic local anesthesia, disuse can be produced without the complication of reaction to nerve injury. Employing unit recording of Renshaw cell discharge can distinguish alterations in the efficacy at the Renshaw cell from those at other sites in the recurrent conditioning pathway.